426 Crate Hemi Dyno Test

Lucky us! We happened to get our paws on a crate Hemi--and the green light to run it to within an inch of its life on the dyno. Even better, we had a chance to bolt on a selection of go-fast goodies to see how much easy power could be netted from the same basic package. Actually, our Hemi is a 426-cube crate engine from Mopar Performance, the smallest in their stable of Hemi crates.

The 426 Hemi crate is basically a turnkey engine, requiring little more than a carb and headers to run. The engines are fairly mild in specs but are rated at 465 hp--a fistful more than the original 426's 425hp rating. Like the original engines, the crate carries cross-bolted main caps and iron Hemi heads, and features a 4.25-inch bore combined with a 3.75-inch stroke. The camshaft is a hydraulic lifter profile with .278 degrees of duration and .495/.480-inch lift, while the valves are the traditional 2.25/1.94 package. Compression ratio is a pump-gas-friendly 9:1.

Bridging the gap between those mighty valve covers is a dual-plane, single four-barrel intake manifold, a departure from the original dual fours of legend. Mopar Performance also offers a longer stroke version of the crate Hemi, at 472 cubes with 525 hp, as well as an aluminum-headed 528ci version rated at a whopping 610 hp.

Hemis are known for one thing above all else, and that is making power. The free-flowing Hemi heads encourage airflow and allow for prestigious power production, even in stock form. While other engine designs need major cylinder-head upgrades to support high horsepower levels, the Hemi responds to high-performance mods with significant output gains while maintaining the basic engine package. Well, that's the theory we wanted to put to the test.

Our plan was simple: Open the box on the stock crate Hemi, add a carb and headers, and spend a day tuning on the dyno to see what kind of power we could extract. From there, we'd make a couple of specification changes via a hot-cam combination, as well as an upgrade in the induction and exhaust systems to see what could be accomplished.

DYNO TIME: BASELINE

Once the lid was cracked on the Hemi's shipping box, we added a complete Milodon dual external-pickup oil-pump package and full-length oil pan, since that configuration will be used when the engine is installed in a car. Next, the factory dual-plane intake manifold was topped with a vacuum-secondary 950-cfm Holley carb, and the Hemi was ceremoniously loaded onto the Westech Dyno.

Once the Hemi was loaded, a set of Hooker 21/8-inch ceramic-coated headers were bolted to the heads, and the factory mechanical water pump was replaced by a Meziere electric unit. There's nothing quite like those gleaming valve covers to draw attention, and the Hemi got more than its fair share. With a flick of a switch on the dyno console, it fired to life, and it was showtime.

7/20

After several pulls on the dyno, the optimal jetting for our 950hp Holley was square at 82 jets front and back. We recorded a best of 486 hp at 5,800 rpm--a good 21 hp over the rated output. Considering the electric water pump, we figured the numbers were right on the mark. We set the ignition timing at 38 degrees total. Next, we ran through a timing loop, surprised to find the Hemi gaining power as timing was pulled out. With a setting of 33 degrees, we recorded a new peak output of 494 hp at 5,700 rpm. When you consider a displacement of only 426 cubes and near-500hp output with a relatively mild combo, it's hard to argue with the concept of Hemi power.

We spent a half day on the dyno with the essentially stock Hemi, and then off it came for additional mods. With the factory combo, we already witnessed enough power to satisfy the needs of almost any power-hungry speed junky. Consider the entire package: mild compression for pump gas, a maintenance-free hydraulic cam spec'd for daily use, and a no-hassle single four-barrel carb sitting on a modest two-plane intake--about as foolproof as a combo can get. We had to wonder: If the Hemi could show these numbers in such a demure package, what would we find if we turned up the heat a bit?

ROLLIN' THUNDER

Turning up the heat pointed us to the camshaft as a starting point, and this time we didn't fool around. We considered a Hemi a solid base for a serious performance mill, and we decided to move straight from a hydraulic flat tappet to a stout solid roller. Solid rollers aren't cheap, but neither are Hemis, so we felt justified. Instead of a full-on race roller, we opted for one of Crane Cams' street roller grinds--a PowerMax SR-254/374-2S-12. This cam is not for the fainthearted, spec'ing out with .254/.262 degrees duration at .050, and .587/.568-inch lift on the intake and exhaust lobes, respectively.

Along with the camshaft, the roller conversion requires the matching roller lifters, PN 66515-16, matching pushrods, PN 66624-16, and a camshaft thrust button, PN 99163, to keep the cam from walking in the bore. Also required with a billet roller is a compatible aluminum-bronze distributor drive gear, which we picked up from Milodon. A roller cam is much more aggressive than a hydraulic flat tappet, and requires a significant upgrade in valvesprings. We swapped out the factory single springs for a matched set of Crane duals (consult the cam manufacturer for recommendations for your application).

We then had a solid street-roller-equipped crate Hemi that was otherwise stock. We suspected some additional changes would be required to make the most of the new camshaft combination, but, at the time, we were content to run the engine with this single change and test it to see what kind of gain it would net. We had a bit of a delay before we could schedule a follow-up day on the Westech dyno. Unfortunately, the 950hp Holley carb we had used for a baseline was no longer available, so we substituted a 975 Race Demon carb on top of the factory dual-plane intake.

The Hemi cranked to life with the sound to match its serious performance looks. The stout street roller had the unmistakable burble of a heavy-hitting performance combo, but it wasn't so outrageous that we were afraid to run it on the street. Again, working the tuning loops, we zeroed in on the optimal jetting, and using the same 33-degree timing setting as in our previous test, we found 534 hp at 6,400 rpm. The cam change was worth 40 hp--a solid gain--and came with a good 700-rpm increase in usable powerband without detracting from power production lower in the rev range. Still, we felt the camshaft/cylinder-head combination should make some explosive power even higher in the rpm band. The key, we believed, would be to complement the combination with a change in the induction system to let it breathe even higher up the range.

With our new combo, the dual-plane factory intake manifold appeared out of place. We wanted a serious single-plane intake manifold that would add to the high-rpm breathing capacity of the Hemi heads worked by the Crane roller cam. To fill that bill, we scored a high-rise Barton single four-barrel manifold. The Barton piece has the unmistakable look of an intake single-mindedly designed for performance, and comes with Dominator-pattern carb-mounting provision. A Dominator or King Demon carb bolts directly in place, while a 4150 standard-pattern carb requires an adapter. We opted to move to a large-bolt-pattern carb--a 995 King Demon.

With the intake and carb change, our Hemi took on a different character, at least in the looks department. The wickedly sweeping divorced runners of the Barton intake and the fat King Demon carb had the look of serious hardware. While we found the appearance satisfying, we were more interested in the numbers. It seemed a shoe-in that the induction was the cork in our previous combo, and confidence was running high that the upgraded induction would let the rest of the combo swill some serious air and turn a number.

We found tuning the King Demon no more difficult than a conventionally sized carb, and we limited our tuning to running a standard jetting loop. The King Demon features replaceable air-bleed circuits; however, no changes were required to get a fine-looking fuel curve. With the induction change, we hit paydirt, recording 594 hp at 6,600 rpm within five dyno pulls. As is typical with a single-plane versus a dual-plane, the lower end of the torque curve softened some, but the high-end charge really woke up.

The intake and carb were worth another remarkable 60 hp, pushing the output of our 426-cube engine so close to the 600hp number we could taste it. We wanted to push the engine over the magic 600hp mark, but our dyno time was winding down. A little more flogging or a change to synthetic lube would have easily moved it over the hump.

Stashed in the corner at Westech was a set of TTI 21/4-inch big-tube Hemi headers. On a 426-cube engine, we suspected they would be a little too much, but at least we gave them a try. Time was short, so we left our oh-so-close-to-600hp previous combo and made the move to the fat-tube headers. Unfortunately, they were a little too much for our combo, and we saw power drop over most of the range, with the exception of a blip above our previous output line in the midrange. It was clear we weren't going to hit our magic number with this combo, but, sorry pal, we were way past the two-minute warning, and there was no time to go back. Too much header cost us torque, power, and time.

So, how do we sum up this little adventure? Well, we can say what Mopar fans have known for years--Hemis make power. We had close to 600 hp on hand, close enough to call it there. When you consider we got there with only 426 cubes and the stock cylinder heads, it walks the walk. Try writing a list of other production engines that will do the same, and you won't have to worry about running out of ink.